Traditionally the turn signal and emergency flasher controls for the incandescent warning lamps of automotive vehicles have employed electromechanical flashers to periodically turn the lamp current on and off to achieve flashing operation of the lamps, although it is also well known to use electronic oscillator circuits to achieve the same purpose. The characteristics of the application places certain demands on the circuit whether it switches by opening and closing mechanical contacts or by solid state switching. The incandescent lamp load has the characteristic of drawing large inrush current when the filaments are cold and their resistance is low and then decreasing to a steady state value when hot. The inrush current can reach ten times the steady state value. This cold filament phenomenon occurs for each cycle of the flasher. Thus, while the flasher circuit is ideally designed for the steady state value, it must accommodate the inrush current as well.
In the case of heavy duty applications, such as a motor coach having many lamps flashing simultaneously, a steady state lamp load of 20 amps is not uncommon and thus current inrush values of 200 amps are to be expected. Electromechanical flashers have been used for this purpose however the switch contacts are subject to erosion which eventually leads to flasher failure. To avoid that problem heavy duty electronic flashers have been proposed using bipolar power transistors which suffer large heat losses requiring large heat sinks. The heat sink for such flashers is so large that the flasher package is very much larger than the electromechanical flasher it is to replace and can not readily fit in the space of the replaced flasher. In addition the large heat output of the flasher is wasted electrical energy.
U.S. Pat. No. 3,875,527, in FIG. 6b discloses an oscillator having a MOSFET to control logic gates in the circuit and thus handles only signal level currents. The oscillator triggers a lamp circuit but does not handle lamp current directly.
U.S Pat. No. 4,185,232 shows a flash lamp operating circuit having a DC source, a DC to AC converter, and a voltage doubler to provide sufficiently high voltage for flash lamp discharge. The circuit also has an astable multivibrator for timing the flashes of the lamps.
U.S. Pat. No. 4,613,847 discloses a flash lamp operating circuit having an oscillator driving a DC/AC inverter which employs a FET and which in turn energizes an AC/DC converter and voltage multiplier which generates voltage for the flash lamp. The lamp is triggered by a separate circuit.